During development of the nervous system, precursor cells divide in specialized proliferative zones, then migrate away from these zones and differentiate. The mechanisms that allow coordinated cell cycle exit and directed migration can readily be examined in the developing cerebellum. In studies supported by this grant, my colleagues and I identified two extracellular factors that regulate migration of cerebellar precursors: stromal cell derived factor (SDF-1 alpha) and brain derived neurotrophic factor (BDNF). We demonstrated that mice with targeted gene deletions of either SDF-1alpha or of BDNF exhibit both aberrant migration and proliferation of neuronal precursors in the cerebellum. In the proposed studies we will define the mechanisms whereby these factors stimulate migration, and how they coordinately influence proliferation. 1. The first specific aim is to determine the mechanism by which BDNF promotes granule cell migration. We will use genetic and pharmacologic tools to identify the signal transduction pathways required for directed migration, focusing on the possibility that TrkB activity stimulates PI3 kinase to promote cell migration. 2. The second specific aim is to test the hypothesis that a gradient of BDNF causes redistribution of TrkB receptors to amplify the gradient and provide a direction for chemotaxis. Preliminary data indicate that BDNF induces an asymmetric distribution of the BDNF receptor, TrkB, with accumulation of receptors at the leading edge. We will test the hypothesis that redistribution of TrkB receptors allows amplification of the gradient, determine whether TrkB redistribution reflects movement of surface receptors or the addition of new receptors to the surface, and determine whether receptor redistribution is needed for chemotaxis. 3. The third specific aim is to test the hypothesis that SDF-1alpha functions as a spatially restricted competence factor that limits SHH-induced proliferation to the EGL. We will investigate the possibility that SHH and SDF function as competence and progression factors for precursor proliferation, and identify transcriptional targets that depend on the synergy of these two factors Taken together, these studies will identify mechanisms that regulate the directed migration of granule cells, and define the relationship between migration and proliferation. Since unregulated migration and proliferation are the hallmarks of malignancy, these studies will provide a basis for understanding and treating brain tumors.